1. Field of the Invention
This invention relates generally to semiconductor programmable read-only-memories (PROMs), and more particularly to a programmable read-only-memory element having a vertical structure.
2. Prior Art Statement
Generally, monolithic semiconductor read-only-memory (ROM) devices are programmed during the manufacturing process, by building the program into the structure of an array of memory elements. Once manufactured, the program in the ROM cannot be altered or changed, since such changes require modifications to the programming masks employed during fabrication. In addition, the cost of manufacturing a ROM to a particular customer's requirements is large and economically prohibitive unless a large number of devices with the same ROM program are manufactured. These problems led to the development of the programmable read-only-memory (PROM) in which the manufacturer uses a standard process for all parts, and the customer programs the individual parts as desired. Generally, such PROMs use relatively large metallic fusible links that are melted or vaporized to create an open circuit. However, the relatively high level of programming current required by such devices demands high-power programming transistors which tend to be both slow and space consuming. Another problem with present PROMs is that the fusible link will sometimes "grow back" and thus change the ROM code.